However, your description of the conversions is completely unclear. Can you post a link to someplace that describes the actual operation?
here is where i got "procedure" for conversion. Its in Greek, i translated through google translation
HOW TO ADJUST THE GPS IN EGSA 87
Many friends ask us how they can configure portable GPS to show the coordinates of the Greek reference system EGSA87. First of all, all new models of portable GPS manufacturers allow the user to input a reference system of their choice. Although the new portable GPS models include over 100 reference systems, the Greek EGSA 87 system is not included due to its very limited use at local and single level. So anyone interested in using road or mountaineering maps that have been compiled with the EGSA87 reference system must enter the parameters in his portable GPS so that there is a proper mapping of the coordinates given by the map to those given by the GPS.
The process of introducing a new reporting system is extremely simple, as long as we have the EGSA 87 parameters. First of all, in the menu (usually on the SetUp screen) we select the option referring to the date-based geodetic system (for Garmin select SetUp-> Units-> Datum) and select User or User Defined. Then our GPS device shows a screen with parameters to which we have to enter the following numbers:
D (a) = + 0.0
D (x) = - 200.1
D (y) = + 73.9
D (z) = + 246.0
D (f) = + 0.00001462 (or + 0.1462 for Magellan models)
Once we set the Date as above, then we must correctly identify the Coordinate System or Position Format-Grid. To do this, we must go back to the SetUp menu and select SetUp-> Units-> Position Format (for Garmin models) and select User or User Defined. Next we have to enter the following parameters:
Projection: TRANSVERSE MERCATOR (if requested, not required on Garmin)
Latitude Origin: 0.00000 (if requested, not required on Garmin)
Longitude Origin: E 024.00.000
Scale (factor): 0.9996
False East: 500000.0
False North: 0
Unit to meters conversion: 1.00000 (if requested, not required on Garmin)
Magellan models allow you to define primarily Coordinate System (Coordinate System) reference and Coordinate Systems. This allows you to keep the main system compliant with international standards, and enter parameters for EGSA87 into the secondary system. You can also choose to read the information by simultaneously displaying both systems.
By entering the above parameters and selecting the User or User Defined Date as the reference system, our GPS will show co-ordinates compatible with the EGSA87 system.