R 如何合并嵌套在另一个数据帧中的所有数据帧并在一个结果数据帧中生成所有组合

我用下面的代码重现了大约sampledata个数据:

###producing sample data
sampledatain1 <- data.frame(in_1=c(1,2,3,4,5),in_2=c("A","B","C","D","E"),stringsAsFactors = F)
sampledatain2 <- data.frame(in_1=c(6,7,8,9,10),in_2=c("F","G","H","I","J"),stringsAsFactors = F)
sampledatain3 <- data.frame(in_1=c(99,98,97,96,95),in_2=c("Z","Y","X","W","V"),stringsAsFactors = F)
sampledata <- data.frame(row = 1:3,
                    colmerge1 = c("MA","MB","MC"), 
                    colmerge2 = -99:-97,
                    stringsAsFactors = FALSE)
sampledata$df <- list(sampledatain1,sampledatain2,sampledatain3)

我想平整所有嵌套的数据帧并将它们组合在一起，使结果看起来像desired_data:

###reproducing desired data 
library(data.table)
sampledatain1$row <- 1
sampledatain1$colmerge1 <- "MA"
sampledatain1$colmerge2 <- -99

sampledatain2$row <- 2
sampledatain2$colmerge1 <- "MB"
sampledatain2$colmerge2 <- -98

sampledatain3$row <- 3
sampledatain3$colmerge1 <- "MC"
sampledatain3$colmerge2 <- -97

desired_data <- rbindlist(list(sampledatain1,sampledatain2,sampledatain3))

如何使用函数从sampledata生成Desired_data呢？