Use eternity's XV2skillcreator, it really helps, but you have to edit the .bac-file (controls animation and camera) and the .bcm-file (controls transformation stages) yourself, skillcreator won't do it for you, it only copies an existing one (perhaps the Super-Saiyan .bac and bcm).

There is a tutorial for the skillcreator: animegamemods.freeforums.net/thread/1745/creating-skill-xv2-creator?page=1
That is your first stage of learning. Be careful what you set as your partset, that is where your transformation starts, per transformation stage one partset up
(i.e. SSJ=297>SSJ2=298...). So either you enable new textures for existing partsets, or you make new ones. This needs to be edited in the .bcs-file.
Editing this is pretty easy. You need genser (also a tool from eternity, you should really check him out, he has some great tools) so you can read the data.
So now you search for an empty slot, i.e. 291-296 should be free, but there are many others you can use. Now you have to decide what you want to change on each transformation stage.
Just copy one of the existing partsets and remove all the slots you don't want be changed when transforming. (For Super-Saiyan you only need the Hair section, if you want spiky hair, if you want nothing changed, you can leave it as it is). Once you created all the costumes in the .bcs-file don't forget to add all required data in the respective folder, use XV2patcher for that (i.e. DB Xenoverse 2/data/chara/HUM). Normally you need following files: .emb, .emd, .emm, .dyt.emb, _LOD01.emd, _LOD02.emd, _LOD03.emd. If one file is missing, your game won't load battle anymore, so it is pretty safe to test it on existing partsets first, if you aren't sure that you haven't done anything wrong.

If you have done all that (if you have an x2m-file with your skill), you have to edit .bac and .bcm-files. So install your new skill, because then you get the .bac and .bcm-file named correctly in your mod data folder (DB Xenoverse 2/data/skill/MET).
Copy both somewhere unrelated and use genser again.
- .bac-file:
Scroll down, as far as possible, then go up slowly. There should be different "bacentry"-sections. You will notice that the last two sections are pretty short, much shorter than the others. Scroll past them until the first long "bacentry", copy the whole section (<bacentry flags="0x0"> to </bacentry>) and insert it directly after the copied one. If you used the skill files of the Super-Saiyan Transformation (3 stages), there should be 3 long "bacentry"-sections, if you want a 6-stage-transformation you need to insert your copied "bacentry"-section 3 times, you need one for each stage. Save it.
- .bcm-file:
Now you need to add your additional transformation stages. There are four parts in the .bcm-file:
(1) First and Last "bcmentry"-section: They function as a bracket, no major changes needed. I explain later
(2) The second to the last "bcmentry" is for your untransform ability, also no major changes needed.
(3) Second to the Fourth entry (if you use Super-Saiyan skillset) is for Transforming into the respective part from normal.
(4) Fifth to the third from the last is for Transforming into the respective part while already transformed.
So now copy the Second one (from the top, SSJ-Skillset: the one where you need 500 KI) and insert it right in front of the copied one. Now you have to make a few changes:
First of all change the required KI into whatever you want to (It should be higher than the stage before, to prevent bugs). Then you need to +1 the two "bac_index values" and the "Trans_modifier value". You notice that the entries are counting down the existing transformation stages. Next step is (4), go to the entry where it all starts again (KI required=500), copy it, insert it before the copied one and change the "bac_index values" and the "trans_modifier value" accordingly.
IMPORTANT: U_18 value indicates if your transformations are possible in general. c80 for transforming from normal (3), d00 for transforming from already transformed (4). So you need both, best in the given order. I don't know what happens if you change the order.
Last but not least you have to change the entry index numbers: <BcmEntry idx="N" ptr_30="M" ptr_3C="N">
N = starts from 0, every entry +1; M = N+1
Change that for every entry you have. Now you have 4 Transformation stages enabled, for more, repeat everything.
Save it and use genser again on both .bac/.bcm.xml-files.
Copy the new .bac and .bcm-file into your folder (DB Xenoverse 2/data/skill/MET) and you are good to go.