I see what you mean... Friggin' annoying! Can't do it Stud.io Ver 1.0 (157) either. I haven't installed Stud.io 2.0 yet, to see if it's possible in the upgrade.
Hmm... You could... Just create a work-a-round that could potentially make your model a little more unique.
These program issues are frustrating, but they're fun when they force us to think of an alternative.
Here, try these Mech Fisticuff Gauntlets. The pictures look like nonsense, so let me explain the layout.
It reads loosely in columns from left to right. Your new fist configuration build are the 4th and 5th Columns.
Final Note: LEGO may not have those Skeleton legs in the colors we want in real life, but look through for other minifig leg or hinge+1x1 plate combination to get your Mech finger/knuckles right.
EDIT=>HERE For reals. You can download my LDD file from my Google Drive, and import the alternate into your build file in LDD. Save you some time.