You should always be able to edit specific fields, especially initiative, in either the group tool or init tool. There is a column called Init (usually, it depends on the game file you loaded) in the group editor to change that value. In IT you would be able to edit the NPC in the index card for it.
If you want to add a creature into the lookup table that is a little harder. I'm working on the editor for that now. Until that is finished you have to modify the file by hand.
Hmm, not sure what I was doing wrong then. I know I definitely couldn't edit STR, DEX, or anything big like that. There was no Init column, but there was a Mod column, I think. That may of been it? The other reason I was asking about the editor, is that I wanted to utilise the auto-calcing HP function on the index card. And again, I couldn't manually edit that from the group editor.
I have worked out the best way to manipulate the group file manually now, have most of the base custom creatures in with their stats, HP tracking and Initiative and all.
Here's hoping InitTool keeps growing!