Retirement tax questions

So you could have a large traditional pre tax Ira balance, and still make a non deductible contribution, convert the non deductible contribution into a Roth without taxes, as long as the traditional pre tax money was moved back into a 401k by the end of the conversion year.   So you don’t have to wait to do the conversion until your traditional Ira balances are zero, just make sure that happens by Dec 31.  -correct?