I have to raise a ten foot 2-1/2 IMC riser about three feet. Building is going from a flat roof to a trussed one.
Can I use a 3 foot piece of PVC between the meter base and the 10 foot IMC?

I sugest that you use a threadless IMC coupling and connector and a short length of IMC to raise your riser mast. This will avoid having a problem with bonding the metal section of the mast.
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Tom H