Yes, you can. There is no requirement to keep them separate, as all your Roth IRAs are treated as one for purposes of determining whether distributions are
qualified. Prior to the Final Roth IRA regulations being issues, it made sense to keep Roth conversion and Roth contributory assets separate, as the five-year ruled applied separately to each Roth IRA- ---and regular contributions could not be made to a Roth conversions IRA (or was it the other way around? it does'nt matter any more) ---