We tried with just the usual (non-fixed) front wheels, and yes, the front axle of the Britax B-Motion and the Britax B-Agile lines is compatible with one and other wheels. It is not a perfect solution, but it can be clicked in, yes.
Fyi, even the rear wheels are compatible, but one needs to swap the internal axles (the small silver ones - to use the B-Motion type axles even with B-Agile wheels if on the B-Motion pushchair and vice versa) as well, otherwise the brake wouldn't work.
With this non-swivel axle, I never tried, but it makes sense it should be compatible. However, the thing is that the B-Agile terrain axle is EVA foam only, so it's going to be lower to the ground than the rear wheels with tires of the B-Motion.
There was, in fact, also a B-Motion fixed-wheels axle available in the past, but not anymore. Maybe it's worth checking in classifieds.
In any case, it should be compatible, yes, but it's not the best fit.