You can only construct expressions in a Conditional Split that apply to the columns in the current data flow. So if you need to do a lookup to see if you can find whether a column in your flow matches something in another table, you'll have to do a lookup or other transformation before your conditional split.

@divyanth - ISNULL() works perfectly in SSIS. The problems I've seen people have with detecting "blanks" in columns is entirely related to their inability to understand the difference between NULL, a string with no characters in it, and a string with only spaces in it. (I'm being blunt - I mean no disrespect.) ISNULL detects NULL values. NULL values are different than an empty string or a string with only spaces in it - so ISNULL doesn't equate those to NULL.