I imagine you want to add credits or something else to the end of your animation, am I right? For that you have some options. I'd say you can create a new Final comp with the desired final time and insert the one you already have done in the beginning and create the final part (or add the final part right after the main one if you already has this final part in another comp). That's the best approach when you have animations splitted in different comps. Think of that as groups of animations that you want to bring together so they need a timeline with enogh time for them to fit.
Another approach is to go to the composition settings from the Composition menu and change it's duration to accomodate the final part. Then just drag de final part comp to the end of your animation and you're done. Please pay attention that when you make your composition duration longer in the Comp Settings the Timeline zoom does not change and you'll need to zoom the timeline out to review the extra time you just added.