I'm the OP of this question. I too had a mix of pre and post tax contributions to my IRA and wanted to separate the two. The only way I know of how to accomplish what you want is to do is to roll your IRA pre-tax portion to a 401K (IF the employer allows it which mine does). What your left with is the post-tax or "roth eligible" portion in your IRA, which is then converted (at zero tax because that's been paid) into your Roth IRA.
But...if you're retired, and no longer have an active 401K (that allows "roll ins" from a TIRA) I don't know of any other account that can serve the same purpose as the 401k does to isolate the pre-tax from the post-tax contributions. You can read more on it here:
Isolating IRA Basis For Tax Efficient Roth IRA Conversions
https://www.kitces.com/blog/roth-ira-conversions-iso[product key removed]ver-pro-rate-rule-employer-...
