Here's your answer.....NOT RECOMMENDED...NO.
The carbon blocks in the RO unit have tremendous surface area while "rocks" of carbon (granulated carbon) have very little surface area. The 1 micron or 5 micron carbon cartridges of carbon blocks aren't there to physically "filter" out impurities with the small holes....the 1 or 5 micron holes are there to increase surface area and chemically bond to the impurities.
They actually sell granulated carbon filters for RO units but highly recommend using carbon blocks because of the tremendously large surface area of these filters. So I originally said no, but you could use granulated carbon, but don't expect it to perform anywhere near to how a carbon block would perform.