Is there a way to export Oracle's UNDO? I tried exp utility to dump all database. Looks like this exports only the last version of data skipping undo log. Using flashback queries I see:

01466. 00000 -  "unable to read data - table definition has changed"
*Cause:    Query parsed after tbl (or index) change, and executed
           w/old snapshot


What I'm trying to do is to capture db changes, make backup for later use with the ability to flashback to timestamp. 

With rman backup I have similar situation:

ORA-01555: snapshot too old: rollback segment number 3 with name "_SYSSMU3_2472002983$"  too small 
01555. 00000 -  "snapshot too old: rollback segment number %s with name \"%s\" too small"
*Cause:    rollback records needed by a reader for consistent read are overwritten by other writers`. 


Update:
I managed to do what I needed only by increasing undo retention and direct copying of data files and control file modification on cloned instance.
You could insert the results of a flashback query or flashback versions query into a table and export that table or you could do your own versioning.  You could also look at the Change Data Capture feature which may meet your requirements.  The real answer to your specific question though is...

No, you can't do that.