Excel custom time format with optional hours

I have a range of data which are in a time format of hh:mm.ss (my original dataset is imported through a csv this way). I need to transform it to a correct format (hh:mm:ss) to make calculations and that's easily done using the replace functionality.

The issue is that some of these data don't have the hh part, they are just mm.ss, so when I replace "." with ":" they become hh:mm instead of mm:ss, e.g. 06m.30s becomes 06h:30m.

Does anyone know how to make a custom time format that will take as default value the mm and include the hh only when necessary?

This assumes that during the import process, the time column is imported as TEXT

EDIT: Formula shortened.

Convert the values to a REAL Excel time

=TIMEVALUE(SUBSTITUTE(IF(ISERR(FIND(":",A2)),"00:"&A2,A2),".",":"))

or

=--(SUBSTITUTE(IF(ISERR(FIND(":",A2)),"00:"&A2,A2),".",":"))

Apply your desired Time format to the result

hh:mm:ss