You cannot. A composite fill applies only to a composite wall, slab or roof (and shell in 15) as the cut fill for one of those elements. It is not a fill that you can draft with, such as in the Profile editor.
So, you either have to reconstruct the skins in the Profile editor, or you can use the commands to 'capture' the profile of an existing wall (or walls) as a new profile .. and then copy the fills from this captured profile to your working profile.
More specifically in your case:
1. Draw a temporary wall using the composite that you need to utilize
2. Select the wall(s)
3. Design menu > Complex Profiles > Capture Profile of Selection
5. A Profile Editor window opens with the captured wall profile - all skins, with total height equal to the height of the selected wall. Horizontal stretch zone created corresponding to the fills marked as structure 'core'. Edit/copy paste these as necessary into your further customized profile.
Cheers,
Karl
PS Note that multiple walls, at different heights and spacing (including air gaps), can be captured this way to quickly get a complex profile started.
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB