Is it really that complicated? Well, it can be, depending on what your two
databases look like. The problem really is, that you are speaking in very
general terms about something that needs to be extremely specific. If one of
the databases cannot ever be changed, then your only option is to make the
other database look as much as possible like the first one. That is, the
tables should look the same, the fields in the tables should be named the
same, the data types of the fields should be the same. Once you have done
that, then you can just import the information from the second database into
the first. If you have tables with different information than the first
database, you will need to find out how to link those tables to the primary
keys of tables in the first database.
It is as if you are saying that you want to remove the door from one of your
cars, and put it on a different car. And you come to this forum and talk
about how all you want to do is move the door, but you don't tell us what
make or model either of the cars is. We cannot help you, except in general
statements, because you are not giving us any specifics of the two databases.
With a car door, specifics would be the size, shape, and type of hinges. In
a database, specifics are things like what the tables are, what are the
fields within the tables, which fields are primary keys, and what are the
data types of the fields.
One thing you might consider doing, if you have ever used Excel very much,
is to place all of the information from both databases into Excel, using
cut-and-paste to match up information as well as you can. Then create a new
database from your website database, with all of the tables, queries and
such, but without the data. Then import the combined data from Excel into
the new database. This way, if there are autonumber fields or such, it will
not matter, since you will have cleaned all that up before the import.
If you have not been doing it, please make sure to do regular backups of
your databases. That way, if you make a mistake that you cannot recover
from, you will have a way to bring everything back.