Your items are kept in the inventory in sequential order as you obtain them. To add an item, find the last item in your inventory and add the new item after that, replacing the placeholder FFFFF...000000....FFFFFF....etc.
For example, items in your inventory in HexWorkshop will look something like this:
400000000000046A000000
63000000
450100000000000000000000004000000000
00046A000000
01000000
46010000000000000000000000400000000000046A0000
00
03000000
47010000000000000000000000400000000000046A000000
6300000
0
48010000000000000000000000FFFFFFF00000000FFFFFFFFFF0 00000000FFFFFFF
0000000FFFFFFFF00000000FFFFFFFF000000000FFFFFFFF00 000000FFFFFFFF00000
where the BLUE is the item quantity and the RED is the inventory sequence number. Since 48 is the last item in your inventory sequence, if you want to add something, you would give it sequence number 49.
So, if you want to add 99 humanity you would replace the first sequence of placeholder FFFFF00000FFFFFF00000...etc. with 40000000000001F40000006300000049010000000000000000 000000.
I added a bunch of stuff last night and it worked fine. Just be sure your quantity and inventory sequence numbers are in HEX. I'm no expert by any means so I hope this makes sense.