Are there tax implications of mixing pre and post tax dollar in Traditional IRA when converting post tax dollars to Roth IRA?

I have Traditional IRA that was converted from 401K employer few years ago. So the money in this account in pre-tax.

I am not eligible to contribute to Roth IRA because of income limitations.

Two questions:

1. Can I put new money which is after tax into Traditional IRA and convert all of that new money  to Roth IRA? Are there tax implication for mixing pre-tax and post-tax money? 

2. If there are implications or of the tax returns are going to be complicated, Can I create new Traditional IRA just for this purpose and then convert money from new account to  Roth IRA? Will it be not considered as mixing pre and post tax money and should not have any tax implications? Will it simplify tax returns?