It should be as direct as:
C:\PS> abc.exe /u c:/programs/abc.dll
However, you may encounter problems with quoting and other characters that are interpreted by PowerShell. Normally a quouting argument will suffice, but if it still does not work, you can use the Start-Process in PowerShell 2.0, for example:
C:\PS> start-process abc.exe -arg @'
...
'@
PowerShell, echoargs.exe args exe. :.
C:\PS> echoargs /u c:/programs/abc.dll
Arg 0 is </u>
Arg 1 is <c:/programs/abc.dll>
Echoargs , EXE.