Apologies for the delayed response. As far as I know, creating custom parameters at account level is not supported. You may check the supported entities per field in this
guide. Additionally, if the custom parameter used has the same name, AdWords will use the most specific custom parameter created. You may check this
article to know more about custom parameters. If in case, you are using custom parameters with
value track, the same rule should be applied (use the most-specific tracking template created, in your case, this should be the adGroup-level).