MD600 - as I understand it, when you apply for your 61.75 certificate, you specify the 'airplane category rating' for which you are applying. Fixed wing and rotorcraft being two separate category ratings, I assume you would need to re-apply for a new certificate to cover a new category on a 'foreign' license.
Nigel H - as specified in the legal judgement letter linked to earlier, in addition to the 61.75 certificate, a pilot is required to have 'an endorsement in his or her logbook as prescribed in 14 CFR 61:31.' You can look up the detail of this requirement
here but it is, in effect "...to have received training...appropriate to the pilot certification level...and an endorsement for solo flight in that aircraft from an authorised instructor."