What would your thoughts be on adding a bedroom and bathroom to this floorplan without expanding square footage?  

You are better off finding a solid property that needs light cosmetic work (paint, floors, or maybe even a kitchen or bathroom remodel), buy it at a decent discount and put 25% down or more (this is when you get best rate terms). 

I've already renovated one of the bathrooms, and I’m planning to paint the interior, update the kitchen cabinets, possibly sand the wood floors, build a fence is a corner lot and increase the rent for the room since my tenant is currently renting the entire 3-bedroom, 2-bathroom house for $800 a month.

Short 1.5 minute video walk through of all of the rooms, bathrooms, common areas, etc. 

They focused on renovations that appeal universally.. think updated kitchens and bathrooms - and kept meticulous records of their expenses.

In the event my landlord is interested in selling, I was wondering if I could add value in the short term by, say, converting part of the garage and/or covered porch into a bathroom and maybe a bedroom (currently 3 bed 1 bath).

Then you ask them to show you their kitchen, bathroom and one or two other rooms (basement or living room).

I turned the extra room without a bathroom into a workout room as a bonus amenity.

Most landlords will use the remodel clause to remove tenants if they dont want to sell.Just replace the kitchen and bathrooms.

Start with a rental that has modest rehab requirements such as updating kitchens and bathrooms and make sure you qualify for permanent financing through a lending source that offers interest rates similar to that of  a bank.