Nope, you can't. You'll need a translation service on the side. The basic problem is that ISIN (or CUSIP) represent a financial instrument, whereas RICs represent a financial instrument on a specific market, traded in a specific currency.
To get the same granularity as the RIC has, you would need more information, basically something like ISIN + MIC + CUR. If you had these three attributes then you would be able to uniquely lookup the equivalent RIC and then use that. (I think, I'm not even sure it's always enough)
As for lookup service (known as cross-reference or symbology service) I'm sure you can buy something from TR. :-)
As Lars has mentioned, if you would like to subscribe to realtime bond data via EMA,
You would need RIC = Reuters Identifier Code.
If you would like to use TR Datascope Select (DSS) for RIC lookup,
You may find the complete documentation the developers portal at:
Java examples illustrating RIC lookup via DSS REST service, named "Search by ISIN" and "Search by CUSIP" are available at: