The rex part you can do with a REPORT and a MV_ADD = true in the transforms I think, however the base64 command is another question, as you can't call commands in props/transforms.
I think you would either have to still have the base64 in your search, or thinking outside of the box a bit:
You could implement a base64 lookup as an external script lookup, although I don't know what the limits are of the size that you can input (i.e. this probably won't work for big base64 blobs)
Are you trying to get the base64 encoded field decoded through the
props.confconfiguration? If so, this can't be done, there is no method of doing the decoding within the
props.conf file. You can extract the field, but it will be in the encoded state, not decoded.