Using blade on supporting unit in A Game Of Thrones
This is clearly stated on page 20 of the rulebook. (For reference the rulebook and the FAQ.)

On p. 20 it states:


  If either the attacker or the defender holds the Valyrian Steel Blade token, that player now has the option of using its ability to provide +1 to his total Combat Strength. If used, flip the token to its faded side, as a reminder that it cannot be used again this round. 


So, no, a supporting player is neither the attacker or defender and so cannot use the Valyrian Steel Blade in that combat.