How to concatenate separately declared modifiers?

You can use Modifier.then

Concatenates this modifier with another.

val modifierA = Modifier.size(100.dp)
val modifierB = Modifier.background(Color.Red)
val modifierC = modifierA.then(modifierB)
Row {
    Box(
        modifier = Modifier
            .then(modifierC)
    )
    Box(
        modifier = Modifier
            .then(Modifier.size(100.dp))
            .then(Modifier.background(Color.Green))
    )
    Box(
        modifier = Modifier
            .size(100.dp)
            .background(Color.Blue)
    )
}

These three modifiers are equivalent, excluding color.

You can use the composed function:

Declare a just-in-time composition of a Modifier that will be composed for each element it modifies

Something like:

Box(modifierA.composed{ modifierB })

or

val modifierC = modifierA.composed{ modifierB }
Box(modifierC) {}