Absolutely! Fabsil is completely safe to use on your convertible's soft-top roof. Make sure that you're only applying Fabsil to clean surfaces, and that you're allowing enough time for the treatment to be fully dry before exposing it to any moisture.
If you accidentally drip or drop Fabsil on to non-absorbant surfaces (including bodywork, glass, and plastic), it will dry to leave a visible residue. If this happens, the residue can be easily removed with a good quality car shampoo.