Ok I don't think you fully understand ... you never pay taxes twice. The amount you rolled from the 401K was all before taxes so when you convert the money to the ROTH it is a fully taxable transfer. Then if you make a non deductible contribution that amount is now a basis in the account. And the amount of the basis cannot be converted to the ROTH in a vacuum which is why what you are wanting to do is not really feasible or allowed.
For instance ... you had 100,000 rolled from the 401K in the IRA and then you put $5000 non deductible contributions in the IRA. At the end of the year you have $105,000 in the account ( ignoring any earnings at this time for this example) and you convert the $5000 to the ROTH then you have to allocate the basis which is what the form 8606 does ... https://www.irs.gov/pub/irs-prior/f8606--2020.pdf
