What are the options on that defined field? Screenshot(s) and other details, please!
I assume when you say "concentrate" you really mean concatenate. What you describe, though lacking in detail as observed by Beverly, sounds a bit like you may be importing into the wrong fields. Check your field mapping carefully.
When the "import options" window pops up, make sure you select "perform Auto-enter options while importing"
The concatenation should happen in a calculated field outside of the targeted fields of the import.
If there is not an extra calculation field to house the proposed merged data a script can need to follow on the found set to perform that concatenation.