3.5 with Compendium Monsters

Is it possible for me to import pathfinder SRD monsters into my vanilla 3.5 games? Do I have to enter them all in?

I don't think there has been anything written(API or a character sheet) to do so.  Someone would have to write an API script, and/or a sheetworker/javascript within the sheet itself that would parse a text-based srd and convert it to be used with a given sheet's attributes. 

Looks like Diana P. is the author of the 3.5 sheet. How much work would be involved? Would it be easier to take the Pathfinder sheet and simply strip out the Pathfinder specific stuff?

I can't speak for Diana.  She has an awesome sheet, but I know it would definitely require some heavy lifting. The community PF sheet has around 40K lines of code.  But you're welcome to have it.   ;-)

The 3.5 sheet does not have any of the code in place to allow imports from Roll20's Compendium because I was going to wait until we had a 3.5 Compendium to use for those imports.  I don't think that the basics of that is too tough; it mostly involves adding additional class and attributes to each of the target input tags/areas.  The main difficulty comes in creating a parser if needed.  My Javascripting ability is slightly worse than my html/css programming and as such is limited to chopping up someone else's code and attempting to frankenstein it into a working kludge.  I don't think it would be appropriate for the 3.5 sheet template in the Roll20 repository to have import codes for the Pathfinder monsters on it since the systems don't work quite the same way.  (though obviously if you have a Pro game and can use custom sheets, you can modify them to your heart's content :) I've thought about trying to include a scriptworker-based JSON importer for monsters for the 3.5 sheet, but I don't know if there is a source for JSON files for 3.5 monsters.  If there isn't, you'd spend as much time formatting the data for the JSON file as you would just copy-pasting the information into the character sheet fields. I don't know of any API-based monster importers that work with the 3.5 sheet either, unfortunately.  The community PF sheet is only 8-10x the size of the 3.5 sheet.  :)  It has a tad bit more in the way of features.  (and a lot of programming that is way beyond my skill!)