The answer to this, once upon a time, would have been no.
If your guitar, violin or other instrument didn't have some form of connection, you couldn't plug it in - you were faced with the sad reality that without investing in a new instrument, you were destined to play acoustically.
Thanks to recent advances in technology, all hope is not lost - enter the Fzone Pickup Microphone.
This clips to the headstock or other convenient place on your instrument and converts the vibrations into sound patterns for the amplifier.
It all sounds a bit far fetched - I was skeptical myself until I tried it, in an attempt to prove the claims it makes wrong - but I was pleasantly surprised at the quality you get for so little a price. I think the main factor with it is that it actually works - just by clipping it on and plugging in to the amplifier, I was able to add effects, change the volume and do everything I would normally with an electro-acoustic instrument.
Obviously you won't get the same quality you would from an instrument with the electronics built in, but this makes a great cost-effective way to add a little extra volume. Perfect for open mics, home use or class leading.