Unfortunately there is no such mechanism in TXDataset. Each new select will simply clear the existing records (close and reopen the dataset) as with any (most?) of Delphi datasets.
I think In a remote system its important to have this functionally. I ve 20000 records on DB but i need to modify or insert only 10 ! my problem is that i must looking a record to server : if found i must get it otherwise insert without loosing old loaded records. If i use mem table its became very hard to modify or insert