I have physical compendiums for my D&D campaign, but I am unable to use the content online

So, I'm running a D&D campaign with my family using Roll20 to ease up on the use of physical things like maps, tokens, etc. Right now, they're all level 2 and will probably reach level 3 in the coming session. However, there's a major issue that I've had to try and work around when I first helped them create their characters: I do not have access to online compendiums. As a result, backgrounds and subclasses are extremely limited. In the charactermancer, I appear to only have the "Acolyte" background online, and for fighter specifically, I am only allowed to choose the "Champion" subclass. I'm sure this is the case for other classes as well however.  While I do not have online compendiums, I have two hard copy compendiums with backgrounds, subclasses, spells, etc: "Sword Coast Adventurer's Guide" and "Player's Handbook". While it was easy getting around the lack of backgrounds (I could just make a custom one and transcribe the details from the hard copy books word for word), it doesn't look like I'll be able to do that for subclasses. Is there any way I could get these compendiums online without having to buy them AGAIN? Like, some sort of code I could redeem from the hard copy? 

Is there any way I could get these compendiums online without having to buy them AGAIN? No, there isn't.

Although the content is the same, the arrangement is totally different and translated into an on-line data structure. It's a huge amount of work. It's like the difference between buying a book and a movie ticket for the same story. Some publishers (like Paizo), have implemented a sharing system with Roll20, but WotC has not.

Andreas J. said: Is there any way I could get these compendiums online without having to buy them AGAIN? No, there isn't. So to have the content unlocked on Roll20, I have to buy it again?

For a certain definition of "buy it again" yes. This is true for anyplace you would buy WotC content. Buying a hardback book does not grant access to it on DnDBeyond or Fantasy Grounds, either, for example. Currently, there is no method of applying the purchase of one toward another.

But, other than convenience for drag/drop functionality there is nothing to stop you manually inputting the values required from your hardcopies. Think of the VTT as a literal (virtual) table top... use the digital character sheets as if they were physical and you had to use a pencil. You can manually add all the required information for subclasses as you did for the backgrounds, it is the same method and section of the sheet, generally :)