If I donated 25 euros during the kickstarter and I want to add to it so I am able to play the Alpha tomorrow, will the money I donate here add on to what I donated in kickstarter so I can get the Alpha?
If so, how do I add to it?
Yes. Once you login to the pledge page and select a pledge you will see the amount of money you need to pledge in order to achieve a certain Tier.
So Baron Digital is $45, you pledged $25, difference is $20 which you would need to pay in order to be known as Baron.
Edit: Not 100% accurate 
The Tiers an Kickstarter were a little cheaper than here, so it might be slightly more expansive than $20. (I pledged Baron Physical via Kickstarter, which now here is $55. So pledging for Earl would be another $55, but I would need to pay $62. Just slightly more as I said.)