You probably want:
q <- data.frame(100,200,200,200,200,200)
colnames(q) <- colnames(SPY)
q <- xts(q, as.Date("2016-01-26"))
# SPY.Open SPY.High SPY.Low SPY.Close SPY.Volume SPY.Adjusted
# 2016-01-26 100 200 200 200 200 200
class(SPY)
# [1] "xts" "zoo"
class(q)
# [1] "xts" "zoo"
tail(rbind(SPY, q))
# SPY.Open SPY.High SPY.Low SPY.Close SPY.Volume SPY.Adjusted
# 2016-01-19 189.96 190.11 186.20 188.06 190196000 188.06
# 2016-01-20 185.03 187.50 181.02 185.65 280016900 185.65
# 2016-01-21 186.21 188.87 184.64 186.69 189174000 186.69
# 2016-01-22 189.78 190.76 188.88 190.52 163849600 190.52
# 2016-01-25 189.92 190.15 187.41 187.64 122676200 187.64
# 2016-01-26 100.00 200.00 200.00 200.00 200 200.00