Andy,
Why would you WANT to do that? If it is just that your Reach operator's certificate is an ITSSAR one, that's no hurdle. An ITSSAR cert can be converted to RTITB by either presenting it at an RTITB refresher (as sufficiently good "documentary evidence of previous training"), or as the basis of a CPD update on your NORS registration.
Both are cheap options; the CPD cost is peanuts.
If it is because you want to use an ITSSAR standard (such as the ability to "reduce the minimum duration" on a course where the customer wants RTITB certs, just DON'T! The NORS applications won't go through, and you'll end up getting grief about making associations with RTITB that they don't accept.