The problem is that what is defined in a .fig will always be a figure, not a panel that can simply be parented by your tab. A figure cannot be parented by anything.
One thing you could try (I haven't done so I'm not sure if it would work) is to launch your figure (i.e. the one that represent a single tab), making it invisible (you can set 'Visible' to 'off' in GUIDE itself.
Then, assuming everything you want within that .fig is in a panel you could try re-parenting the panel from the .fig to your uitab. This would probably involve some unpleasant use of findall or similar to fish the panel handle out of your invisible figure, but it might work.
I would suggest creating the simplest test case you can to see if this works rather than on your real code. Just create a .fig with a panel in it and some slider or whatever and then create your uitab on command line or in a script and try locating the panel handle from the launched figure and reparenting it.
Personally I tend to do programmatic UIs when I use tabs, but it is a big inconvenience (though GUIDE performance is so bad nowadays it is less of an inconvenience than it might have been!)
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