Hi Wilson.
Yes, you can connect an external clock source to either TMR0 or TMR1 on the 16F877A.
If, however, you're talking about connecting a 32.768kHz crystal directly then only TMR1 has that capability (it's got an internal oscillator circuit specifically for this purpose). TMR1 is 16bits wide so you'd get 2 overflows per second. A prescaler of 2:1 will allow you to fit your 800ms in one go.

/Henrik.