If you have admin access to the software that generates the RSS feed, you have a lot of options as detailed here. If you don't have that admin access, or want an easier solution, the only working ones that I found are in this list.
Working and probably working solutions:
- Feedburner. It is proposed that one can onvert a simple RSS feed into one that has PubSubHubbub enabled in it by feeding it through feedburner.google.com [source1, source2]. This normally works. With one exception: If the feed uses HTTPS (and only that; as done by n-1.cc for example), this triggers a longstanding bug in Feedburner, namely "Received HTTP error code 400 while fetching source feed." [source1, source2].
- feedblitz.com. This seems to be one of the best Feedburner alternatives. (I did no make exactly sure though that they do indeed add PubSubHubbub when processing one feed into another one.) The drawback is, it always costs money. However it costs money only for the publisher, not for the subscribers, and you pay only 1.49 USD monthly for any number of RSS subscribers if you do not have e-mail subscribers.
- Self-hosted PuSH hub with polling. There are free software PubSubHubbub hubs you can run on your server. Hopefully one included polling – start searching here. However, compared to the lightweight utilities "RSS to Identi.ca" and "feed2omb" (see above), this seems like overkill. Except you want PuSH feeds anyway.
Partially working and non-working solutions:
- superfeedr.com and Guzzle Ayup!. It is possible to create both a free publisher and subscriber account at superfeedr and Guzzle Ayup!, which results in a RSS feed with PubSubHubbub enabled [source]. However, subscribers to this feed have to use a username and password, and pay for getting notifications. This is a problem, as such a fee is only acceptable for big subscribers like a social network app operator. For normal users (or the Identi.ca SubMirror plugin for auto-posting RSS entries), this is not a viable solution.
- Open PuSH hub with polling. If you find a gratis PubSubHubbub hub with pollling, that would work. It is like the superfeedr.com solution above, just that accessing the feed does not need climbing over a paywall. I did not find such a service though, maybe you have more luck.
- Zapier. Does not work: You can create a RSS-to-RSS zap there, but the target RSS has no added PubSubHubbub support. So Identi.ca cannot subscribe to it. Also it is a HTTPS feed as well, so we can't route it through Feedburner to add the PubSubHubbub …
- IFTT. Does not work: With them, it is not possible to create a "RSS to RSS" chain at all. Somebody tell 'em.