Can I have a tank fed water heater in line with a tankless water heater?
The problem is probably not the tankless these heat the water extremely fast, the real problem is probably the size and length of pipe to the point of use.  A tanked water heater will probably take a similar amount of time to get the water hot to 120 at the point of use, what I have done is installed a small tanked unit after the on-demand with 2 check valves and used a recirculation pump at the furthest location cycling the water through the loop with a timer. One check valve prevents reverse flow through the loop and 1 to protect the tankless. We did insulate the pipes were accessible and this worked once we got the cycle on off time adjusted, I added the check valve between the 2 to protect the tankless with the tankless kind of isolated from the small tanked heater there were no false turn on's on the tankless (recommended by the mfg). This method will increase your electric usage but the small water heater 7 gallons was enough to keep the hot supply on a ~3500 SF ranch home with 3/4" water lines. Recirculation systems are not bad when you have large lines that are long but because all the lines are kept warm it cost more but the good news is it can be done. On smaller homes I have installed 2 point of use electric units 30 amp. But with larger homes the combination of a large tankless and a small tanked system with a recirculation system is much much cheaper.