QuickBooks does not have a built-in method for combining data files. There are some options:
If you just need to move data from one file to another, there third party tools to transfer data available at marketplace.intuit.com.
If you have remote offices that only need to add certains kinds of data, such as quotes, you can use a third party tool that integrates with QuickBooks to record your quotes and then syncs back to the many file. There are many different applications on the marketplace website that offer this sort of functionality.
If you have remote offices that need full access to your main data file, you have the following remote access options:
Terminal Server combined with the Enterprise version of QuickBooks.
QuickBooks Online Edition, which allows you to connect from any Internet-enabled location
QuickBooks Pro or Premier combined with remote access software.
Make sure you save the file as a tab delimited text file. Just taking an Excel file and changing the extension to .iif isn't enough, you actually have to change the save format. Click on File->Save As and change the type to Text (Tab Delimited), but change the extension from .txt to .iif. If there is still a syntax problem in your IIF file, at least you'll get an error message. When you import from the XLS format, it just says that the data is imported, because it isn't finding any data in the correct format.
This is a known issue caused by the addition of the Sheet reference to the formulas that get exported to Excel. Here is a KnowledgeBase article that explains in more detail.
This has been fixed in the R8 update which can be downloaded here.
The IIF format does not support creating links between transactions when you import. However, there are third party tools to help with this available at marketplace.intuit.com
Yes you can transfer and import your customer, vendor, chart of accounts, and many more lists from one company file to another. While in your QuickBooks company you would like to export from, go to "File", then "Utilities", choose "Export" and "Lists to IIF files". A box will pop up for you to check off which lists you would like to export. The system will default to save these files in your QuickBooks folder. Once completed, open the company file you would like to import them into. Go to "File", "Utilities", "Import", "IIF Files". Your QuickBooks folder will open, and you should see the files that you had exported. You will need to import these one at a time. Depending on what you need to import, there is a specific order in which importing should be done.
You can import lists from Excel, that presents you with import options: You can overwrite the existing information with the new data, have it prompt you to make a decision when it sees a duplicate item, or ignore any changes it sees.
I would make a backup of your file before attempting this.
Go to File>Utilities>Import>Excel Files for this feature.
There is not any functionality within QuickBooks to "combine" data files. There is a product that offers the ability to convert a QIF (Quicken Export File) to an IIF (QuickBooks Import File) from Big Red Consulting. I did not see it on the marketplace.intuit.com add-on site but you can learn more about it here.