Import custom post table to wordpress and keeping IDs

I am standing in front of a problem I can’t really see how to resolve. I am about to transfer a website which is written in PHP to wordpress. There’s really not much complicated about this but one thing. The website has near 30k posts which contain only a title and an image (much like e.g. 9gag). These posts are stored in a MySQL Database and the urls look something like this:
domain.com/parameter/id
with ID being the ID from the database. It is crucial to keep those urls on the new site for facebook likes/comments and Google search results. However those ids are for the most part incrementing by one each. On WordPress each post I create has 4 consecutive IDs in the wp_posts table, this way blocking the IDs for the three next posts. As far as I am seeing it, the additional IDs are used for further data concerning the post (e.g. image attachements). However in the old database each entry had a value containing the path to the attached file. Is there any way to realize this the same way in a wp_posts table? To have a custom field with the image path that wordpress would recognize to show the image in the post? I am new to wordpress, so I am still trying to figure a lot of things out. If you need any further information please let me know. Please respect that for privacy reasons I can not give out the domain name and exact urls.

Best Regards, Jay