Assuming the stain is easy to remove, and you are unsure of the type of sealant that was used on the floor before, it is possible to put an epoxy coating over stained wood. However, because stains can be difficult to remove completely, this may not be ideal. If you decide to go ahead with this approach, make sure to use a professional epoxy contractor who can help you target the area of the floor that needs protection, and who can also advise you on how to remove any residual staining afterwards.