Yes, you can define a SAS LENGTH or better an ATTRIB statement declaring the variable at some point in your SAS data-input processing flow (likely a DATA step that performs the input handling or one just after a PROC IMPORT). That should be enough - I would not recommend coding a RETAIN statement either.
This approach presumes you have standard definitions maintained for your incoming data-field structures, to mean that the length or type attribute (CHARACTER or NUMERIC) does not change between processing cycles.
Scott Barry
SBBWorks, Inc.