The simple answer is: NO! Repacking a valve which is under pressure can cause personal injury and/or property damage, and should never be done.
Repacking a valve is part of routine maintenance. Valve packing forms a pressure seal to keep the process media (water, steam, other) from leaking. Over time, the packing will wear, relax, or loosen and begin to leak due to media temperature, seasonal ambient temperatures, or even valve cycling. Media that leaks out of a valve can be costly, messy or even harmful to health and environment.
Repacking a valve is not a difficult or lengthy process. Proper preparation, with trained personnel and factory-provided packing made specifically to fit your valve should make the process go very quickly. When done correctly, service interruption will be minimal… minutes, not hours… of system down-time.