I don't believe so, at least not generally, some metrics have a matching (min and max) threshold metric so you can graph both, but not all metrics have this.
This should be raised as a RFE - or check the RFE list, I think this may have been raised before.
The file is here. You need to delete the extension '.txt' and copy it into the "CAS/config" directory.
No need to restart the service, the user defined metric will appear immediately in your DMI metrics.
For more detail, look at the doc User Defined Metrics:
If I understand your question correctly - you should be able to use the "max slow operation threshold" or "min slow operation threshold". These are the max (or min) load time thresholds for what you are displaying.
If you have a single URL with a Page Load Threshold then both values will be the same. Likewise if you have a software service where you set the threshold they will be the same.
If you have a software service with a Page Load Threshold and URLS with thresholds set (in URL Monitoring) - these values will be the max or min of all thresholds set with in the software service or on URLs in the software service.