I would just start Helping out...

Tell the person who is currently doing the Jon that you have some down time and would live to help him out if he needs it. if He says yes have him assign you some tasked in a unofficial way.

You need to be very careful with this, only do this if you KNOW that you are doing a good job at what you have been officially assigned and that your boss is happy with it. If not, he WILL get annoyed that you are focusing on other stuff and may blame that for the lack of quality...

Your primary job Takes priority.

I would just start helping out...

Tell the person who is currently doing the job that you have some down time and would like to help him out if he needs it. If he says yes, have him assign you some tasked in a unofficial way.

You need to be very careful with this, only do this if you know that you are doing a good job at what you have been officially assigned and that your boss is happy with it. If not, he WILL get annoyed that you are focusing on other stuff and may blame that for the lack of quality...

Your primary job takes priority.