Usually no: a standard softbox is generally designed for strobe or flash heads with a single bulb and a specific speedring/adapter mount. A rectangular fluorescent fixture with multiple tubes is already a relatively large light source, so it produces softer light than a bare point-source strobe to begin with.

Because of that design, a normal softbox often won’t fit or work properly on this type of head. If you want even softer light, a front diffuser or a modifier made specifically for that fixture type is the more likely solution. Avoid improvised coverings on continuous lights unless they’re intended for the unit, because continuous fixtures can generate heat.

Also note that continuous fluorescent lighting and strobe-plus-softbox setups don’t behave exactly the same: strobes give better control over ambient light and light spill, while continuous lights let you see the effect directly but may be less efficient for some uses. So before buying, it’s worth deciding whether a continuous light or a strobe system better matches your needs.