It kinda is, using the 'find fixture' option in patch mode.
Press patch>dimmer>options>activate 'Find fixture', press exit.
Then you can use wheels to select dmx line and dmx channel. Selected channel will be set @ 255.
(NB : Using this option while patching fixtures will locate the selected fixture)
This is of course, however, temporary (selecting another channel or exiting patch mode will set your channel back to 0) and, afaik, you cannot chose a custom value for this (i.e. '127', or '50%').
Out of this, no, it is not possible to light a fixture out of the blue without patching.
You cannot, for example, use your avo to test channels 2, 6 and 9 by sending mixed and simultaneous dmx values without patching.
You can use an expensive DMX tester for that - or just patch the damned channels
