I see no reason why not. You can certainly (ex-LON, not sure about with connections) book a WT/WT+ base ticket no problem and MFU'ing at the same time (rather than online after) is generally best in case the online system tries to throw some extra change fee/charges [beyond the extra fuel surcharges which are valid] in as it sometimes does.

Way back when it used to be the case that you could only MFU any sector if all sectors were eligible to MFU, but as far as I know that went out the window long ago as confirmed here by the likes of Bukhara or others a few times.