Hello,
since I have not fully read up on the A310 yet, please verify.
On all Airbusses I know, at the END OF PLAN, you get the PLN to ALTN. This you can build as per your OFP. However, this is not possible it seems on the A310, because the ALTN PLN has no start point. Say for the flight I attempted to do today. LPPT/TNCC. The ALTN was SVMI. You build your flight plan, which ends at the final missed approach waypoint. Under that line, it correctly states END OF PLN. However, the ALTN part only has SVMI in it. As such there is no way to build a route. I can't insert anything before SVMI. And selecting a STAR (let's say the ILS to 10L) yields no waypoints. So, a flight plan to alternate cannot be built. On other MCDU's I know, you at least get a discontinuity before your ALTN so you can build the plan up.