Replacing a "tag" in a Variable with Value from Another

I have a parameter file in CVS format that I process within powershell scripts. The file is in the format of:
VariableName, Value
Several of the variables include the path to specific files. The 'root' of this path is always the same, and in one instance is used several times in a variable (it is actually the commandline parameters for an SSIS package, executed with DtExec.exe.

What I would like to do is have something like:

ROOT,C:\SomeFolder\SomeFolder2\SomeFolder3\HomeFolder
PACKAGE_CMD_LINE,/FILE "%ROOT%\MyPackage.dtsx" /CONFIGFILE "%ROOT%\MyConfig1.dtsConfig" /CONFIGFILE "%ROOT%\MyConfig2.dtsConfig"  /WARNASERROR  /CHECKPOINTING OFF  /REPORTING EWD /CONSOLELOG NCOT 

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in my parameter file. Within Pwershell, it will set the values of $ROOT and $PACKAGE_CMD_LINE to the appropriate values. Then, within the code, I want Powershell to replace the '%ROOT%' "tags" within the $PACKAGE_CMD_LINE variable with the value of the $ROOT variable, which has already been processed. Can someone assist me as to how this might be accomplished.
Subsequently, I would have a line that reads:

& DtExec.exe $PACKAGE_CMD_LINE

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But $PACKAGE_CMD_LINE would have had occurances of '%ROOT%' replaced with the value of $ROOT.