Uhhm, I am unsure about what others will say, but your post, at least to me, appears a bit like time-machine stories! I am not sure what to say, it's like if a high school student realises suddenly that they've missed out on pre teen or even early teen hobby clubs, and wants to re do all of that in high school (which has its own joys and sets of clubs and equivalent stuff)..I am sure technically nobody's gonna stop you from joining Hall activities and other such undergrad-dominated stuff, but will you really like it?
I mean, at my uni, we PhDs avoid the undergrad haunts like crazy and hang out at our little pub..yes full of geeks but there's no geeky talk there LOL! Also, I hope you want to do a PhD for Phd-ish reasons and not cos you want to be at school for all the reasons you just said...
Well, I dont know what I am saying you could do, but definitely, you can technically join everything, but it's perhaps gonna feel and look a lil out of the usual, cos most such organized activities are usually pretty much undergrad and perhaps new post grads dominated....you will end up missing out on PhD peer activities, and then, when wil you make THAT up? PhD life demands other commitments, publishing, presenting, staying in touch with the latest research, seminars, hanging out with fellow researchers, research talk etc...
I guess you should do what this stage demands...be a doctoral student, and enjoy the trials and joys of the whole thing...its not bad. Trust me. :)