Are you allowed to use a Metamagic Rod (say Extend) with a spell cast from a scroll or other magical device?
No, unless the GM rules otherwise of course. "The modifications made by these [metamagic] feats only apply to spells cast directly by the feat user. A spellcaster can't use a metamagic feat to alter a spell being cast from a wand, scroll, or other device."
And metamagic rods just let you act as if you have the feat a couple times a day.