Here is a formula that you can use in a spreadsheet to convert overs to balls:
=INT(A1)*6+MOD(VALUE(A1)*10,10)
A1 is the cell where the overs are stored in this example.
It works in two parts:
The first part takes the INT() of the overs - which effectively truncates the part-overs by rounding down to the nearest whole number. It multiplies this result by 6 to get the ball contribution from the whole overs (assuming 6 ball overs).
It then adds the part-overs balls bowled by multiplying the whole thing 10 to move the decimal point to the right by one and then takes the modulus 10 of the result (which gives the remainder after dividing by 10).
This formula will work regardless of whether your overs are saved as numbers or stings. It is based on 6 ball overs, but if you have other over sizes then replace the "6" in the formula with your balls-per-over value.
A copy of the .xlsx example: