how to install add-ons that stopped installing after a system update?

The surveillance software that was available for the ReadyNAS is no longer supported.  The version of Jessie it's OS is based on is also so old that generic Linux surveillance packages installed on it won't be up to date, so likely won't support the most recent cameras.  Add to that that it makes no sense to me to have a surveillance system without a monitor, and I just can't recommend using the ReadyNAS for surveillance.

Blue Iris can be run on some micro-PC's, which draw little power.  By using your NAS as the archive location for it, you don't really need a whole lot of drive space on it, either.  There are, I believe, surveillance packages that will run on one of the later generation Raspberry Pies and similar devices, which also draw little power, but I have no experience with them.

Here is my best endorsement of Blue Iris:  It sent the perp who burglarized my house to prison for 39 months and got me restitution for what my insurance didn't cover (but don't get me started on why he didn't have to reimburse my insurance company, too).  After his lawyer saw the video, he took a plea bargain.  The lawyer knew it would hold up in court because it had proper timestamps and anti-tampering code.