JuMP sums use standard Julia generator syntax. So you can go:
julia> using JuMP
julia> m=Model()
Feasibility problem with:
* 0 linear constraints
* 0 variables
Solver is default solver
julia> @variable(m, x[1:3])
3-element Array{JuMP.Variable,1}:
x[1]
x[2]
x[3]
julia> @constraint(m, sum(x[i] for i in 1:3 if i != 2) == 1)
x[1] + x[3] == 1