[[stage_in]] doesn't support packed types, but you can get the same effect using the properties of your vertex descriptor's attributes. For example, if you wanted [[attribute(0)]] (which might represent position) to be laid out as three packed floats followed immediately by [[attribute(1)]] (which might represent a surface normal), also consisting of three packed floats, you'd just set the formats and offsets correspondingly, ensuring that your buffer data is packed as described:

vertexDesc.attributes[0].format = MTLVertexFormatFloat3;
vertexDesc.attributes[0].offset = 0;
vertexDesc.attributes[0].bufferIndex = 0;
vertexDesc.attributes[1].format = MTLVertexFormatFloat3;
vertexDesc.attributes[1].offset = sizeof(float) * 3;
vertexDesc.attributes[1].bufferIndex = 0;