Imported columns fall into two categories:
The data you have that corresponds to PRESTO's known column names should be imported if possible. You should be somewhat more circumspect about importing your group-specific columns:
On the other hand, it is easy to add or remove group-specific columns in a later import. But since IMPORTS ARE DESTRUCTIVE; you can only do a re-import when your import data is up to date.
You have three choices. When you make your choice, one thing you might want to consider is that PRESTO provides a command to export the people in its database to a tab-delimited file. See the Export People command for details.
The first is to continue to view your original database as the master people database. The second is to view the PRESTO database as your master database, but still reflect changes to it into your original database as an insurance policy. The third is to view the PRESTO database as your master database and let your original database become out of date.
Your policy choice can influence how you go about cleaning data during your first import. With the 1st two policies, "clean import data" means all the values in the import file should be properly formatted. With the 3rd policy, it just means the columns should obey the general rules. You can clean individual fields using PRESTO's Update Registration command instead of editing your data base, if you prefer.
If you decide to maintain your original database, you need to fill in Configure Group->Email for DB Changes with who should reflect PRESTO Register Person and Update Person commands into the original database. Setting this field causes PRESTO to automatically send a message describing each change that occurs.
If and when you create a new import file, it needs to contain all the changes that were emailed to the DB-Changer since the last import. Otherwise those changes WILL BE OVERWRITTEN BY OLD DATA when you do the import.
After you import a database that has group-specific columns, it is normal to then use Customize Registration Form. In particular, you need to position the associated fields appropriately within your registration form. (Because of the tabular nature of import files, fields are called columns when importing is discussed. More exactly, the value in the column's header row is the field name and the cells below are the values placed into that field for each imported person).
Customize Registration Form can also be used to add fields to the form. In general though, you should consider doing so only if you have chosen to not use your original database as your master database. However there is one practical exception: you need some new fields immediately and the owner of your master database is temporarily unable to add them.
As noted above, your situation may call for periodic re-importing of your people. You can also use Import Page to add a bunch of people whenever you want (e.g. after a membership drive).
Finally you can do bulk updates as well as real imports. For example, suppose you are not maintaining your original database and you needed a new registration field whose initial value was other than Blank. You could just do a large number of Update Person commands, but it might be easier to create and use an import file that contained the desired initial values.