Downloading the .srt files is a bit of a gray area, as far as discussions go on this forum, but we do support people who wish to work with dvds that they have purchased.
To do this process, you'll need to decrypt and rip the original dvds to the hard drive. I suggest using either AnyDVD or DVDFab for this. There are free trials for either of them.
Once you have the movie and the subtitles on the hard drive, you'll need to import them both into an authoring program. This is where it gets a bit tricky, since many of the free authoring programs want elementary streams from the dvd as input, and won't accept the .vob files seen in dvd-video.
There are many, many different ways and different tools for this process. I'll go through one way for you and we'll see if this gives you an acceptable result. Download Vob2Mpg. http://www.videohelp.com/tools/VOB2MPG You'll need Net 2.0 as well, if you don't have it, but there is a direct link to it at that page.
Run the dvd-video through Vob2Mpg to get one big mpeg file from the dvd. It will not degrade the quality of the movie, but you will lose the original menu and all the extras. You're only getting the main movie here.
Next download AVStoDVD. http://www.videohelp.com/tools/AVStoDVD Install it and open the program. You'll want to make some adjustments to the settings, so click on Preferences. Click on the Video tab and set DVD Video Standard to PAL (since you are in Greece). Also, it is very important to place a check mark in the box for Keep Mpeg2 DVD Compliant Video. Click on the Audio tab and put a check mark in the box to keep compliant audio streams. Click on OK.
Now you have to decide what size to make the dvd. If the original dvd main movie is small enough to fit onto a single layer dvd, you can set DVD Size to DVD5 (4.7gb). If it is larger than that, you can use AVStoDVD to reencode to fit that size, or you can set output to DVD9 and produce a movie without compression that will have to be burned to a double layer dvd. Your choice here.
Look in Output in the main window and make sure it is set to DVD Folder Structure.
You are now ready to import the mpeg2 file you made with Vob2Mpg. Click on the green + sign on the right side of the window in AVStoDVD and navigate to the mpeg2 file. Once it shows up in the main window, click Ctrl S and a new pop up window will appear. This lets you navigate to the .srt files on your hard drive and import them to your dvd-video you are making.
You can make a menu in this program if you want, but not necessary.
Click on DVD Menu, Menu Wizard to do that.
Set your Output folder on the hard drive, make a label for the dvd and then click Start. If you aren't using compression, this should be a very fast process, and there will be no degradation of the quality of the original dvd. If you need to reencode it to fit a single layer dvd, there will be some quality loss, but you may be hard pressed to see it.