what is the meaning of export and import in pole

Normally you can't import directly into an existing album. However, with the Applescript titled Import Folders of Image Files into Library as Albums you can select a folder of photos and imported them into the library and a new album. If the photos are in a folder on the flash drive you can do what you want. Other scripts are available at the Photos for Mac User Tips webpage.

You can download a compiled version of the script as an application from this tutorial site: P01 - Applescripts from Photos’ User Tips Compiled as Applications.

Import all, images to Photos first. They will be sown in an album named Recently Added. Select all the images there, then add to the album you want them in.