The new 1.25 release lets you backup your list of "Bagged" hills to your sd card, and restore from this file or similar. The CSV file that is used has a very simple format - so if you already have a huge list of Bagged Hills, and you already store the British Hills Database id for those hills, it's pretty simple to produce a CSV file that you can restore into the British Hills app.
The CSV file format is as follows:
Hill id,Date Climbed [YYYY-MM-DD],Notes
Note each field should be surrounded by single quotes, e.g.:
'2','2008-3-14','Lost my packed lunch'
'3','2009-6-20','Weather changed. Broke arm in scramble'
If there are any other particular formats that you would like me to be able to import, let me know and I'll have a look.