The general rule is 5 hp per foot of rotary cutter. You say "mower" so I'm not sure if you mean a rough cut (brush mower) or a finish mower. Your BX is 17 pto hp, so it would be way under powered for a rotary cutter and somewhat underpowered for a finish mower.
It depends on how thick and high the grass is. For light cutting, you could get away with it, but for heavy, overgrown grass, you would have to go very slow. OTOH, I have a 5' belly mower on my B2710 (20 pto hp) and it does fine. But adding a foot of cutter and reducing the hp by 3 would be pushing it, probably.
The other thing you might check is the weight and the lift ability of the BX. You might exceed the 3pt lift, or at least need added front weight to keep the front on the ground.