If possible, use IFC. Both programs can handle that. I don't know how to do it though or how compatible it is though. Check help files and docs.
If you just want basic graphics transfer, this might work:
In my experience, Archicad's DGN exporter gives too little control. Use DWG as the intermediate format and the Keep Index Number translator to get control.
Then place all the objects you want in an otherwise empty Archicad .pln file. Save the file as a DWG and check that they become DWG blocks.
If you want them in 3D you'll have to save from the 3D window.
In Microstation, import the DWG and use some utility that saves the blocks as Microstation library parts.
Please note that this conversion isn't painless.
Archicad's library parts are basically 'intelligent' scripts in the GDL language that generate graphics when used in a project. They may also contain any amount of other information, text, images, IFC, cost info etc.
Microstation cells (if that's what you're referring to) are basically collections of graphic entities that may have any amount of additional intelligence (depending on the application) added.
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