Well, if I understand you correctly, you may need to use a concat statement. So your command may be something like this:
concat("CopyTable /dsnamesrc=MySharePointList /tablesrc= /dfs:myFields/dfs:dataFields /rowsrc=d:SharePointListItem_RW[d:Title='", FieldWithValue, "'] /tabledest=/my:myFields/my:group3 /rowdest=my:group4 /empty=yes")
So, let's pretend FieldWithValue is in another data source and the value is Test. The above concat would send a command that actually has the value of:
CopyTable /dsnamesrc=MySharePointList /tablesrc= /dfs:myFields/dfs:dataFields /rowsrc=d:SharePointListItem_RW[d:Title='Test'] /tabledest=/my:myFields/my:group3 /rowdest=my:group4 /empty=yes
Does that make sense?