Some NAD83 based transformations are preconfigured in most Bentley products to automatically apply depending on the coordinate systems selected and to reproject to. For example any geographic coordinate system that is based on NAD27 to be reprojected to another geographic coordinate system based on NAD83 will result in the application of datum transformations based on NADCON. Likewise for a GCS based on NAD83 to another GCS based on NAD83/HARN will get a datum transformation using the HARN specific NADCON files.
Although pre-configured these transformations can be modified or even deactivated by editing the 'grid data catalog' (located in the ‘GeoCoordinateData’ sub-directory of the product) files Nad27ToNad83.gdc for NAD27-NAD83 transformations and the file Nad83ToHarn.gdc for NAD83 to HARN. These are text files and list the NADCON or NTv2 format grid shift files provided by US or Canadian authorities (Canadian files cannot be distributed by Bentley Systems but can be obtained free of charge from the Canadian government). These files are only present for MicroStation v8i and other products of the same generation.
Some documentation concerning geographic coordinate system is provided with MicroStation and additional information can also be obtained from Bentley Map documentation mostly concerning user-defined geographic coordinate system definitions.
MicroStation CONNECT edition uses a different mechanism based on a different format. The syntax is more complex but provides far more flexibility to redefine any datum transformations. The way to configure/add grid shift files in MicroStation CONNECT edition has changed significantly as a result of major changes in the third party reprojection library used. Now it is possible to override any datum definition by adding uncommented entries in a user override file. Provided with Microstation CONNECT edition (and other product of the generation) in the product ‘GeoCoordinateData’ sub-directory you will notice the presence of a file called UserOverrideGeodeticTransformSeed.asc. The beginning comments explain how to create a file that must be named UserOverrideGeodeticTransform.asc to which users must add the required override entry. The seed file contains commented all entries that currently make use of grid shift files. It is possible to simply copy the current entry to be modified to the new UserOverrideGeodeticTransform.asc file and uncomment then modify the entry.

For illustration purposes the entry below is the definition of the ED50-IGN.ES datum transformation as delivered with MS CONNECT modified to add a new file. If new files must be added simply add a GRID_FILE line pointing to the new file as the NEW_USER_FILE that was added. Any number of files can be added an as usual former entries have precedence over later entries.

GX_NAME: ED50-IGN.ES_to_ETRF89
SRC_DTM: ED50-IGN.ES
TRG_DTM: ETRF89
DESC_NM: ED50 to ETRF89 via Grid File (Canadian NTv2 format)
SOURCE: Instituto Geográfico Nacional, Spain
EPSG_NBR: 0
INVERSE: Yes
MAX_ITR: 10
CNVRG_VAL: 1e-09
ERROR_VAL: 5e-08
ACCURACY: 1.0
METHOD: GRID_INTERP
GRID_FILE: NTv2,Fwd,.\Spain\sped2et-2005.gsb
GRID_FILE: NTv2,Fwd,.\Spain\BALR2009.gsb
GRID_FILE: NTv2,Fwd,.\Spain\R2009V9.gsb
GRID_FILE: NTv2,Fwd,.\Spain\NEW_USER_FILE.gsb
FALLBACK: ERP50-SP-EST99_to_WGS84

See the file UserOverrideGeodeticTransformSeed.asc for additional details.

I hope this is sufficient information for your purpose.