As far as I know, there is no way to go from RSEM to RPKM. Is there a specific reason you prefer RPKM over RSEM? RSEM should give expression estimates that are just as good or better than RPKM. From the RSEM paper:
The second measure of abundance is the estimated fraction of transcripts made up by a given isoform or gene. This measure can be used directly as a value between zero and one or can be multiplied by 10^6 to obtain a measure in terms of transcripts per million (TPM). The transcript fraction measure is preferred over the popular RPKM  and FPKM  measures because it is independent of the mean expressed transcript length and is thus more comparable across samples and species .