There is a Ventra customer service location at the CTA Headquarters at Lake & Jefferson (actually, in a smaller office space off to the side on Jefferson, which is part of the larger building.) If one is going to come to Chicago more than once in any 18 month period, especially, I would recommend going there and signing up/registering a new card. Then you'll have it available when you return - and without the $5 registration penalty. Apparently, if the card is inactive for an extended period (18 months or more?) then there is a monthly maintenance charge which will start to drain any value which you have remaining stored.
Passes and value can be added at the Ventra machines, in stations, and at other locations. I just added a 30 pass at a CVS the other day. They used to sell you a mag stripe card. Now they use a Ventra tap machine connected with their register which allows the pass to be added directly to your card.
Important to know is that Ventra will automatically use any passes you have stored on its card in descending order (30 day, 7 day, 3 day, 1 day) before deducting individual fares. You don't get to choose any other ordering. So, for instance, if you wanted to use a 7 day pass which is on the card, but not start it until Monday, paying only for the individual ride you happen to use Saturday, it would be best to have a second Ventra card available for this purpose (you get to name them for online tracking.)
At the moment, Chicago Cards and mag stripe Transit Cards can still be reloaded, but at limited values. I think you can even still buy a mag stripe card at a station (though not use it as a day pass of any sort.) CTA has announced that there is now (again) a planned phase out transitioning, as they have accepted Ventra's standards as finally working and acceptable. However, I am unaware of any details of this yet.