Can I create settlements without workstations?

A settlement is defined in the game as having a Workshop. The Fallout 4 Wikia Page details the information on settlements and settlement building, as well as a list of settlements that can be used. The list is as follows:

Vanilla Fallout 4

• Abernathy Farms
• Boston Airport
• Bunker Hill
• The Castle
• Coastal Cottage
• County Crossing
• Covenant
• Croup Manor
• Egret Tours Marina
• Finch Farm
• Graygarden
• Greentop Nursery
• Hangman's Alley
• Home Plate
• Jamaica Plain
• Kingsport Lighthouse
• Murkwater Construction Site
• Nordhagen Beach
• Oberland Station
• Outpost Zimonja
• Red Rocket Truck Stop
• Sanctuary Hills
• The Slog
• Somerville Place
• Spectacle Island
• Starlight Drive-In
• Sunshine Tidings Co-op
• Taffington Boathouse
• Tenpines Bluff
• Warwick Homestead

Automatron

• The Mechanist's Lair

Far Harbor

• Longfellow's Cabin
• Dalton Farm
• National Park Visitor's Center
• Echo Lake Lumber

VaultTec DLC

• Vault 88

As of writing this on August 9th 2016, these are all the available settlement workshops. It is possible that NukaWorld will add settlements as well.