Alternately just wind the FCU to 100 feet above your current altitude, VS +100 ft / minute and when the aircraft goes to CLB mode you can alter the FMGC CRZ LVL before returning the FCU to the original ALT and VS back to that level. The whole thing can be done in +/- 50 ft. Just be vigilant with your FMA's to make sure you are back in ALT mode at the end of it all.