Automatic Character Set Conversion with R3trans 
The character set conversion is automatically converted for transports between SAP Systems with different code pages.
You can also use this
R3trans function separately. For example, this is a good supplement to your own migration and conversion programs because they are independent of different character sets in the different R/3 Systems.The only prerequisite is that the file you create corresponds to the R/3 transport format. You can then convert the file to another character set with
R3trans and continue processing the file with your own program. As a side effect, packed fields are converted into a valid R/3 presentation.Use the following control file for this task:
convert
input file = ...
output file = ...
If you omit one or both of the file specifications,
trans.dat is used by default at the relevant point. If your operating system allows reading and writing of the same file, input and output files can be identical. This is the case in UNIX and Windows NT systems.The call
R3trans -c <file1> <file2> does the same job.
The character set conversion is currently implemented so that data is always converted into the character set that is valid on the operating system in which
For programs for migrating application data from R/2 to R/3 Systems, the character set conversion has been extended with an exit for special characters.
This extension was required because characters used in R/2 were not always converted into the correct code page (for example, for Danish characters) and this resulted in incorrect character representations in R/3.

You want to change the character Ö (hex d6) to
convert
input file = ...
output file = ...
substitute 0xd6 0xd8

The hexcodes must always refer to the character set.
You can also use the option
SUBSTITUTE for normal imports.