Critical tablespace usage UNDOTBS (98%) This tablespace is in my case 8GB. I'm on oracle 10g and got critical warning:

Tablespace usage critical: UNDOTBS (96%) 
I also executed this command:

select sum (bytes/1048576) MB
FROM
DBA_SEGMENTS
WHERE
tablespace_name = 'UNDOTBS' ;
It gives me 7625,625MB.

Could I extend this tablespace with this command:

ALTER TABLESPACE "UNDOTBS" ADD DATAFILE '/path/to/ts.db' SIZE 4194304000;
Can I do it on running system and wont I destroy anything?
Most probably this is fake alarm. the space UNDO tablespace is allocated differently then on other tablespaces. Oracle simply does not clean out old versions of block unless necessary. There are no real data in this tablespace.

You UNDO space was really full you would get ORA-01555 Snapshot too old, or other types of errors. Check the view v$undostat.