We have a line established to take messages that we would like transferred to a cell phone that is not currently part of the system. Is there a way to transfer from the in system analog phone to an outside system phone (cell phone) or can the cell phone be added to the system in place of the analog phone?
When you say you have a "line established to take messages" do you mean you have a separate telephone number just for this purpose or is it just an extension off of an internal phone switch?
If you have a separate phone number you might want to look into a Google Voice (GV) account. With that you may be able to port the existing number to GV rather than create a new number for messages. Presently GV promotes porting your existing "cellular" phone number over to GV but they may offer the same for actual land lines, I've not tried to do that myself so you might need to ask them.
Here is a link for one of the GV web sites with more info.
Once the number is transferred to GV you can direct inbound calls to ring any phone number, or multiple phone numbers at the same time. You can even set it up based on the time of the day, so it only calls the cell phone at night for example. The other great benefit is that GV will email transcriptions of the message to you so you might not even need to listen to the actual voice message.
If on the other hand you have an existing extension on your phone system making your voice recordings you can still create a GV account and tell your existing phone system to forward all calls to the GV phone number that gets created. From there the same GV voice features will be available and with inbound callers being none the wiser.