When it has been created a connection between a LUW to LUW federated server, the columns section on Alter Nickname windows allows you to modify the information of the columns.
But, if there is another type of federated connection, for example LUW to mainframe, the columns section on Alter Nickname windows it's empty.

The federation code in Toad currently only supports column information on LUW to LUW type Federated connections.
Toad has the ability to define nicknames for other types of connections, however the column support is only for LUW to LUW connections.
WORKAROUND:
Execute the alter nickname sentence directly from the editor instead of the Alter Nickname windows.
For example:
ALTER NICKNAME <SCHEMA_NAME>.<TABLE_NAME>
ALTER COLUMN <COLUMN_NAME>
LOCAL TYPE CHARACTER(5);
COMMIT;
STATUS:
Waiting for fix in a future release of Toad for DB2.