RasterLayer and SpatialPixelsDataFrame objects joint layer problem

[Leprechault]

I want to add a new layer to a SpatialPixelsDataFrame object using a RasterLayer object. Here's an example:

   # Packages
   library(gstat)
   library(sp)
   
   
   # Example data set in SpatialPixelsDataFrame format
   lat <-  c(-23.49174, -23.49179, -23.49182, -23.49183, -23.49185, -23.49187)
   long <- c(152.0718, 152.0718, 152.0717, 152.0717, 152.0717, 152.0717)
   pH <- c(8.222411, 8.19931, 8.140428, 8.100752, 8.068141, 8.048852)
   sample <- data.frame(lat, long, pH)
   
   x.range <- range(sample$long)
   y.range <- range(sample$lat)
   
   x<-seq(x.range[1], x.range[2], length.out=20)
   y<-seq(y.range[1], y.range[2], length.out=20)
   grd<-expand.grid(x,y)
   
   coordinates(sample) = ~long+lat
   coordinates(grd) <- ~ Var1+Var2
   gridded(grd) <- TRUE
   
   proj4string(sample) <- CRS("+proj=longlat +datum=WGS84")
   proj4string(grd) <- CRS("+proj=longlat +datum=WGS84")
   
   dat.idw <- idw(formula=pH ~ 1, locations = sample, newdata = grd, idp = 2.0)
   
   class(dat.idw)
   # [1] "SpatialPixelsDataFrame"
   # attr(,"package")
   # [1] "sp"
   
   # Now I'd like to add a new layer to the SpatialPixelsDataFrame using another SpatialPixelsDataFrame
   lat <-  c(-23.49174, -23.49179, -23.49182, -23.49183, -23.49185, -23.49187)
   long <- c(152.0718, 152.0718, 152.0717, 152.0717, 152.0717, 152.0717)
   temp <- c(20, 21, 22, 23, 24, 25) 
   sample <- data.frame(lat, long, temp)
   
   x.range <- range(sample$long)
   y.range <- range(sample$lat)
   
   x<-seq(x.range[1], x.range[2], length.out=20)
   y<-seq(y.range[1], y.range[2], length.out=20)
   grd<-expand.grid(x,y)
   
   coordinates(sample) = ~long+lat
   coordinates(grd) <- ~ Var1+Var2
   gridded(grd) <- TRUE
   
   proj4string(sample) <- CRS("+proj=longlat +datum=WGS84")
   proj4string(grd) <- CRS("+proj=longlat +datum=WGS84")
   
   dat.idw.2 <- idw(formula=temp ~ 1, locations = sample, newdata = grd, idp = 1.5)
   dat.idw.2 <- raster::raster(dat.idw.2)
   class(dat.idw.2)
   class(dat.idw.2)
   # [1] "RasterLayer"
   # attr(,"package")
   # [1] "raster"
   
   # Now I'd like to add dat.idw.2 to dat.idw
   dat.idw$temp <- as(dat.idw.2, "SpatialGridDataFrame")$v
   str(dat.idw)
   # Formal class 'SpatialPixelsDataFrame' [package "sp"] with 7 slots
   #   ..@ data       :'data.frame':	400 obs. of  2 variables:
   #   .. ..$ var1.pred: num [1:400] 8.05 8.05 8.06 8.06 8.07 ...
   #   .. ..$ var1.var : num [1:400] NA NA NA NA NA NA NA NA NA NA ...
   #   ..@ coords.nrs : int [1:2] 1 2
   #   ..@ grid       :Formal class 'GridTopology' [package "sp"] with 3 slots

But the new layer (temp) in @ data is not added to the SpatialPixelsDataFrame object. How can I add the new layer to the original SpatialPixelsDataFrame object using a RasterLayer?
Thanks!!