All you have to do is register your ITSSAR qualification with RTITB who will then add it to your remit. A CPD should make registering it on NORS ok. If in doubt speak to Nick Welch at RTITB.
As RTITB & ITSSAR are both members of the ABA2012 Workplace Transport it should be ok as they recognise each others qualifications. As for the ITSSAR standard allowing shortened courses this is not true. The only way ITSSAR & RTITB allow the shortening of a course is on in house training only where the training day can be reduced by upto a maximum of one hour per day - this is the case for both awarding bodies

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.