Yes you can. You don't even have to worry about STP because this will not form a loop at Layer 2. What do you expect from this configuration? If you are doing that for redundancy, the problem is that you cannot put two interfaces in the same ip subnet on a router... You would probably end up bridging between the router interfaces and using a BVI. And STP would then be back in the picture. It's not a big deal, but bridging on routers is made in software, and generally inefficient compared to the hw bridging provided by your 2950 so I don't think it's an interesting option.
You can also put one of the two router ports as a backup of the other (in that case you'll be able to have the same subnet on both interfaces, but only one will be available at a given time).
At last, you can also form a ring connecting the last 2950 to the first 2950 in your chain. If you have not touched any STP parameter (in particular, if you did not disable STP;-)), everything should be fine. I would recommend setting the root bridge on the switch connecting to the router. Then, for redundancy, you can still use the backup interface trick to add a second interface between this switch and the router.
Regards,
Francois
