At least it should be possible to assign Ctrl+Tab, Shift+Ctrl+Tab (see bug 86404). No other combinations make sense, IMHO.
Ctrl+Tab behaves just like Tab and Shift+Ctrl+Tab does nothing, so seems like it should be possible to customize them when the cursor is in the document area.
This was fixed in
tdf#86404 - use Ctrl+TAB to switch between tabs/sheets
Now Ctrl+Tab and Ctrl+Shift+Tab are present in the keyboard customization.
*** This bug has been marked as a duplicate of bug 86404 ***