You can use UDIMagic to update existing Master records in Tally including updation of User-defined Fields (created using TDL).
Example:-
Assuming, you already have StockItem Masters data in your Tally Company. You have now created a new field (say MyField1) in Tally (StockItem Master) and want to populate this field the data that you have in MS-Excel.
Sample XML tags for UDIMagic:-
<MASTER TYPE="STOCKITEM">
<!-- Specify the Column which contains the StockItem Name or PartNo -->
<OLDNAME COLUMNREFERENCE="A"/>
<!-- Column B contains the data that is to be populated into the User-defined field MyField1 -->
<UDF:HARVCHSUBCLASS.LIST DESC="MyField1" ISLIST="YES" TYPE="String">
<UDF:HARVCHSUBCLASS DESC="MyField1" COLUMNREFERENCE="B"/>
</UDF:HARVCHSUBCLASS.LIST>
</MASTER>