1. Can I merge two databases? I have not found anything about this, so I guess it is not possible?

Alternatively,
2. What is the best way to copy all the items in one database to the other? At a minimum, I would like to retain the Fields and possibly Tags. If I could also copy over the Grids, that would be great.

From a recent post, Pierre suggested to use XML Copy / Paste.
I tried:
1. I copied the items from one Grid, but not all field values were copied over. It appears that the fields must exist in the receiving database for this to work? Is there some way to have the missing fields created when pasting?

2. NOTE: It is important to expand all items to show the subitems first, so that they can also be selected. Otherwise, only the parents are copied.

3. I then selected a different Grid and copy/pasted into the receiving database. However, because some items in this Grid were also in the previous gird, duplicate items were created.
I guess that I must somehow add all items in the database to one Grid (I could dbl-click on the "Item" field in the properties pane to collect all items in a scratch Grid) and then copy all the items in one shot from there. Can IQ handle copy/paste of several thousand items this way? (How much RAM do I need?)