I would like to specify the exact lead source - is that possible?

I would like to specify the exact lead source - is that possible?

When entering a contact in CRM:

If I select "Conference" as the lead source, I would like to be able to specify exactly which conference, or if I select "Law Firm", I would like to be able to select from a list of law firms that I have entered previously. Is that possible and if so, how can I do it?