Can the word 'genius' be used as an adjective?
For example: 'A genius plan' or 'This is a genius piece of work'?
The simple answer is, yes.
This usage of 'genius' as an adjective is relatively new and would still be considered by many (including me) as slang. In that vein, the following would also be acceptable:
my genius girlfriend
Your plan is genius!
However, it would be more standard (and advised in formal settings) to use the word 'ingenious' in all of these examples, including your own, as it is an adjective.
an ingenious plan
an ingenious piece of work
my ingenious girlfriend
Your plan is ingenious.
As a note, I imagine that the use of 'genius' as an adjective has in fact arisen as a mistaken combination of the two words 'genius' and 'ingenious' in much the same way as the erroneous 'irregardless' has come from 'regardless' and 'irrespective'.
Also, not to be confused with ingenuous.
I'd just point out that "ingenious" is not the adjective form of "genius," and the meanings differ significantly. While using "ingenious" in the above instances would make sense, try substituting the word "clever" or "inventive." That would be a good approximation of what you'd actually be saying. When you think of the word "ingenious," think of the words engine or engineer--they have the same Latin root. "Genius" has a different root.