If your data is in text format (rather than a date/time serial number) then you can add it with a worksheet formula like:
=A1+("00:" & A2)
In this formula, A1 looks like h:mm:ss and A2 like m:ss
If you want the formula to figure out your data type, then try:
=IF(LEN(A1)-LEN(SUBSTITUTE(A1,":",""))=1,"00:" & A1,A1)+IF(LEN(A2)-LEN(SUBSTITUTE(A2,":",""))=1,"00:" & A2,A2)