Can I add a current keeper to any decoder (generically)? I have an old decoder that wasn’t designed with specific current keeper connections.I have a new current keeperII 810160. Is there a way to generically add a current keeper to any old decoder? or does the decoder have to have connections specifically designed for current keepers?
It is possible to do so, providing you can access the positive connection to the power supply and the ground connection. Some may label the connections. If you have a decoder from a German manufacturer, the positive connection will often be labelled as U+.
The DCCWiki has a page on Energy Storage which has more information and links to other sites which can offer more details.
It should be possible with any decoder, but how easy, that is another issue. 
This can be a shaky thing. Theoretically, yes, you can add a current keeper to any decoder. BUT you need to know where to solder it on if the decoder was not designed for one.
I was recently doing this for a client of mine and I came across an old NCE decoder in an F unit. I thought there was plenty of room, even in N scale because of the tiny keep alive TCS has now.
I contacted NCE and one of the techs was kind enough to send me a pic of the decoder where he “thought” was the best place to solder it in. It was a difficult solder job and in the end, did not work. It fried the decoder. Was it my soldering? Well, I have 45 years of experience. Was it the wrong spot? Well, maybe the tech took his best guess.
Some will say it’s easy, just solder it in where the bridge rectifier is, but sometimes that’s hard to determine.
Most newer decoders come with wires or a soldering pad to connect a keep alive. I’ve done a lot in N scale on ESU decoders.