Can I still contribute to an RRSP after converting it to a RRIF?

Generally, no. Once you've reached the mandatory conversion age and matured your own RRSP, you generally can't make new contributions to an RRSP in your own name, since RRSP contributions are tied to being under that mandatory age in the first place. Converting to a RRIF is itself a sign that you've reached the point in the Income Tax Act's framework where your own RRSP contribution room effectively stops being usable.

There's an important exception worth knowing about, though: a spousal RRSP contribution to a younger spouse's or partner's plan may remain possible even after you've converted your own RRSP, because that contribution is based on the younger spouse's eligibility to hold an RRSP, not yours. This distinction between your own RRSP contributions, which stop, and contributions to a younger spouse's RRSP, which may continue, is a real and easy-to-miss detail, and it's worth confirming carefully with your financial institution or a tax advisor based on your specific ages and circumstances, since getting it wrong either way, assuming you can't contribute at all or assuming you still can in your own name, can cost you a planning opportunity or create an unwanted tax issue.