You can convert your old characters to make them compatible with this mod.
The general idea is to save your character as a so-called "json file". This is just a text file following specific "grammar" rules.
Your action bar is an Array looking like this "[slot1, slot2, slot3, slot4, slot5, slot6]" so all you have to do is to extend this array up to "... slot18, slot19, slot20]" .

You need:
- A backup of your storage folder! (You just can't have enough backups)
- A good text editor (geany or vi.... oh Windows? Ok, Try Notepad++ )
-
This steam guide written by fabio.capela for bk3000's inventory mod because it explains almost everything you'll have to do.


How to patch:
1. Back up your storage folder
2. Follow step 2 and 3 of fabio's tutorial
3. Open your .json file with your text editor.
4. You want to find the following line:
"customBar" : [[[{
5. Delete this line and all up to the following line
}]]],
The next line (which you should NOT delete) is
"customBarGroup" : 0,

6.Your file should now look similar too this

or like this (because you wearing a head item)

It's important to keep the },"customBarGroup" intact because of JSON grammar, ya know?

7. You then paste the following line right above the customBarGroup-line:
(Yes this is one single line! up to and including the ]]], part!)

8. Your end result should look like

or respectively:

9. Save your json file
10. Follow step 5 in fabio's tutorial to convert your edited json file to a .player file