That should be possible. You can probably figure out how to do it by checking how other mods did the same thing for skeletons. Check out this mod and compare it to the original files:
http://www.runicgamesfansite.com/torchl ... s/node/196
Which embersentry.dat file are you editing?
I edited the embersentry.dat in "affixes/skills" so the duration for summon duration is -1(which is basically telling the game no value or no limit in this case). It worked at least for rank 1of the skill.
NAME:EMBERSENTRY SUMMON DURATION ACTIVATION:DYNAMIC DURATION:-1 TYPE:SUMMON DURATION SAVE:true MIN:0 MAX:0 [/EFFECT]
Thank you! That seems to do the trick. I was editing the one in skills/alchemist/embersentry.