Is it possible to create a code Interface?

It is possible. Here's a Roblox wiki article on OOP: http://wiki.roblox.com/index.php?title=OOP

For inheritance, define a class instantiation function in terms of a superior class. Here's an example:

vector={
	new=function(...)
		local components={...}
		local length=#components
		return setmetatable({class="vector"},{
			__index=function(t,k)
				if type(k)=="number"then
					return components[k]
				end
			end,
			__newindex=function(t,k,v)
				if type(k)=="number"then
					if type(v)=="number"then
						components[k]=v
						return
					else
						error("Must be a number")
					end
				end
				error("Cannot set "..k)
			end,
			__add=function(a,b)
				if a.class==b.class and a.length==b.length then
					local t={}
					for i=1,#a do
						t[i]=a[i]+b[i]
					end
					return vector.new(unpack(t))
				end
			end,
			__sub=function(a,b)
				if a.class==b.class and a.length==b.length then
					local t={}
					for i=1,#a do
						t[i]=a[i]-b[i]
					end
					return vector.new(unpack(t))
				end
			end,
			__tostring=function()
				return table.concat(components,", ")
			end
		})
	end
}

vector3={
	new=function(x,y,z)
		local object=vector.new(x,y,z)
		rawset(object,"magnitude",math.sqrt(x*x+y*y+z*z))
		rawset(object,"class","vector3")
		local meta=getmetatable(object)
		meta.__add=function(a,b)
			if a.class==b.class then
				return vector3.new(a.x+b.x,a.y+b.y,a.z+b.z)
			end
		end
		meta.__sub=function(a,b)
			if a.class==b.class then
				return vector3.new(a.x-b.x,a.y-b.y,a.z-b.z)
			end
		end
		meta.__index=function(t,k)
			if k=="x"then return x
			elseif k=="y"then return y
			elseif k=="z"then return z
			end
		end
		local old=meta.__newindex
		meta.__newindex=function(t,k,v)
			if type(v)=="number"then
				if k=="x"or k=="y"or k=="z"then
					if k=="x"then
						x=v
					elseif k=="y"then
						y=v
					elseif k=="z"then
						z=v
					end
					rawset(object,"magnitude",math.sqrt(x*x+y*y+z*z))
					return
				end
			end
			return old(t,k,v)
		end
		return object
	end
}

local v=vector3.new(12,23,34)
print(v.x)
local a=vector3.new(23,12,1)
local b=v+a
print(b)