Hi Debbie, thank you for your question regarding Electrical Nonmetallic Tubing (ENT) and proper grounding to metallic boxes. The short answer to your question is yes, ENT can be used for connecting to metallic boxes, fixtures, and junctions, provided that the required ground conductor is solidly bonded to the metallic object in question. In fact, it is quite common to see ENT routed to a metallic light fixture (the ground wire in the ENT raceway must be bonded to the metal frame of the fixture).
In general, ENT is intended for use as exposed wiring or as a concealed wiring method, and therefore must have a ground wire run in the raceway. Often with metallic tubing such as EMT or rigid, an effective ground fault current path is considered to exist under the National Electric Code (NEC) and as such, ground wires are often excluded from the raceway. This exclusion of a ground wire is sometimes permissible under the NEC, however it is often not permissible, and is never advisable. It has been our experience that the impedance of metallic tubing (of all types) is poor and should not be considered reliable for proper Over Current Protection Device (OCPD) operation.
It sounds from your question, that EMT (metal) is being replaced with ENT (plastic), and that the old metal junction boxes are being left in-place and re-used. NEC Article 362.60 requires that a separate equipment grounding conductor be installed in the raceway in compliance with NEC Article 250, Part VI, for all ENT installations. The metal boxes are acceptable, as long as they are properly bonded to ground. One issue of note is that isolated ground systems would require two (2) ground wires; one for the isolated receptacle, and one for the metal box.
Some other issues to note regarding ENT is that if the building is higher than 3-floors above grade, than the ENT must be installed with a 15-minute finish fire rate thermal barrier, and it must be supported at 3-ft intervals.
We hope you find this information helpful, and good luck!
The E&S Grounding Solutions Engineering Team