Joe,
based on the way you phrased the question, I'm assuming you want to add horns while playing live? Your question could be read either way, and as Mario already said, the answer would be quite different depending on what you want.

One way To "approximate" the sound of a whole horn section live:

1) use a guitar synth such as GR-55 to play the trigger note for the horns

2) unless you want all trumpets or all sax, you'd want to use a patch that includes a whole horn section. This would give you the full range of sounds.. but they'd all be playing all the parts, which isn't what a horn section would actually do.

3) run the output of the guitar synth thru a harmonizing pedal. If you don't already have one, Ebay, Craigs list and music stores have many to choose from. Most of these pedals let you set a key for your harmony, and also which notes in the scale you want to include in your harmony.

4) 1st, 3rd and 5th notes with the correct key set and a good horns patch will be a ton of fun to jam around with. Then its up to you to play in a way that captures the nuance of horns

https://www.dropbox.com/sh/ey71h8ep047758h/AAC12jNM0Hz9Q8zsh5iDeqNHa?dl=0